There is a need in practice for thermal-shock-resistant materials which are suitable for use at temperatures of .gtoreq.1000.degree. C. and which, at the same time, have a strength level of .gtoreq.40 MPa which provides for engineering constructions, such as: in melt metallurgy, for example, for throughflow controllers, in machine construction, for example hot gas fans, in engine construction, for example thermal insulations of the exhaust gas port (port liners), in chemical engineering, for example as filters or catalyst supports.
Although ceramics based on pure aluminum titanate, or tialite, show interesting properties, such as a low thermal expansion coefficient (TEC) and a low Young's modulus, they are of only limited technological value on account of their very poor strength and their tendency to decompose at temperatures in the range from about 900.degree. to about 1300.degree. C. At temperatures in this range, tialite decomposes into the starting oxides Al.sub.2 O.sub.3 and TiO.sub.2, accompanied by a marked increase in the TEC.
Numerous proposals have been made for the production of ceramics based on aluminum titanate. Thus, U.S. Pat. No. 2,776,896 relates to a non-decomposing, thermal-shock-resistant aluminum titanate ceramic of which the improved properties are achieved by additions of iron, magnesium and silicon.
According to U.S. Pat. No. 2,776,896, the addition of 1 to 2 mol-% Fe.sub.2 TiO.sub.5 to Al.sub.2 TiO.sub.5 is sufficient to produce substantial resistance to decomposition and up to 50 mol-% Fe.sub.2 TiO.sub.5 may be added without the low TEC being significantly affected. For a ceramic of 90 mol-% Al.sub.2 TiO.sub.5 and 10 mol-% Fe.sub.2 TiO.sub.5, a TEC (RT-400.degree. C.) of -2.35.times.10.sup.-6 l/K is measured after sintering.
In addition to iron, silica may be added to the composition. If silicon dioxide is added, it should be added in the form of clay for practical reasons. The silica content of the mass should not exceed 10% by weight. The formula is preferably calculated in such a way that one additional mol TiO.sub.2 is added for 2 mol SiO.sub.2 (column 7, line 21). This means an excess of free TiO.sub.2 which results in inadequate strength. Thus, all the Examples disclosed show relatively high contents of free oxidic components.
EP-B 133 021 describes an aluminum titanate/mullite ceramic consisting of 60 to 75% by weight Al.sub.2 O.sub.3, 15 to 35% by weight TiO.sub.2 and 1 to 16.5% by weight SiO.sub.2. In another embodiment, 0.5 to 5% by weight Fe.sub.2 O.sub.3 and/or 0.5 to 5% by weight rare earth oxides are also added. The quoted Examples described compositions of the sintered ceramic comprise mullite contents of 20 to 40% by weight, Al.sub.2 TiO.sub.5 contents of 50 to 70% by weight and Al.sub.2 O.sub.3 contents of 10 to 12% by weight. In addition, oxides of iron, lanthanum and neodymium are added. Adequate strength values of &gt;40 MPa are only obtained at sintering temperatures of or above 1500.degree. C. and by addition of expensive rare earth oxides.
EP-A 210 813 describes an aluminum titanate/mullite ceramic, in the production of which at least one of the two components aluminum titanate or mullite is presynthesized and then sintered at temperatures in the range from 1500.degree. to 1700.degree. C. The overall composition is 53-74% by weight Al.sub.2 O.sub.3, 14-33% by weight TiO.sub.2, 6-20% by weight SiO.sub.2 and 1.2-5% by weight Fe.sub.2 O.sub.3. Due to the necessary presynthesis, this process is relatively expensive. In addition, the ceramics produced by this process show inadequate strength values.
De-PS 2 741 434 describes an aluminum titanate ceramic which, in addition to 2 to 13% by weight SiO.sub.2, contains 0.5 to 10% by weight rare earth oxides and 1.5 to 20% by weight of SnO.sub.2. This ceramic shows inadequate strength values.
According to DD-B 29 794, high thermal-shock-resistance is produced by a very low, preferably negative, linear thermal expansion coefficient. For the production of a highly refractory oxidic material showing high thermal-shock-resistance, this publication proposes compositions of MgO-Al.sub.2 O.sub.3 -TiO.sub.2 or of MgO-Al.sub.2 O.sub.3 -TiO.sub.2 -SiO.sub.2, the TiO.sub.2 content being said to amount to between 15 and 75% by weight, the Al.sub.2 O.sub.3 content to between 70 and 25% by weight and the contents of SiO.sub.2 and MgO up to 40 and 20%, respectively. The obtainable coefficient of linear thermal expansion is said to be &lt;4.times.10.sup.-6 l/K in the range from 10.degree. to 700.degree. C., preferably being negative or differing only slightly from zero. The mixing ratios of Al.sub.2 O.sub.3 to TiO.sub.2 shown in the Examples encompass the broad range from 1:0.7 to 1:1.7, the latter range applying to a silicate-free composition containing 8% by weight MgO.
Corresponding materials are of little value on account of their poor strength.
The safe use of the ceramic in practice presupposes a number of properties which the material is required to show in a reproducible form. For many applications, as for example in melt metallurgy, thermal-shock-resistance is an important criterion. Not only a low TEC, but also a low Young's modulus and high strength are crucially important to thermal-shock-resistance. The shrink in behavior of the ceramic is another crucial property in cases where a metal jacket is to be cast around a ceramic tube insert, as for example the exhaust port liners in cylinder heads. The ceramic has to yield to the solidifying and shrinking metal sleeve without breaking. This requires a ceramic material showing high fracture strain in combination with small shrinkage backstresses, i.e. a low Young's modulus and high strength. In addition, the use of the ceramic, for example as a hot gas fan, requires excellent decomposition resistance above 900.degree. C.